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May 31, 2020, 06:55:17 am

Author Topic: very nice misconceptions - lasers  (Read 564 times)  Share 

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very nice misconceptions - lasers
« on: August 05, 2008, 04:50:32 pm »

more specifically for the topic of light and matter:


It's incorrect to say that "in laser light the waves are all in phase." When two light waves traveling in the same direction combine, they inextricably add together, they do not travel as two independent "in-phase" waves. The photons in laser light are in phase, but the WAVES are not. Instead, ideal laser light acts like a single, perfect wave.

When the light wave within a laser causes atoms to emit smaller, in phase light waves, the result is not "in phase" light. Instead the result is a single, more intense, amplified wave of light. In-phase emission leads to amplification, not to multiple in-phase waves. If the atoms' emissions weren't in phase, the result would NOT be light that's out of phase. Instead the atoms would absorb light rather than amplifying it.

Each atom in a laser contributes a tiny bit of light, but their light vanishes into the main traveling wave. The light from each atom strengthens the main beam, but loses its individuality in the process. 99 plus 1 equals 100, but if someone gives us 100, we cannot know if it is made from 99 plus 1, or 98 plus 2, or 50 plus 50, etc.

All the *PHOTONS* in a single wave of light are in phase. This might be one reason that people say that laser light is "in phase" light. However, in-phase photons are nothing unique, and they don't really explain coherence. Any EM sphere-wave or plane-wave is made of in-phase photons. For example, all the photons radiated from a radio broadcast antenna are also in phase, but we don't say that these are special "in phase" radio waves, instead we just say that they are waves with a spherical wavefront. Even if all the photons in laser light are in phase, it is still incorrect to say "all the WAVES are in phase." Photons are not waves. They are quanta, they are particles, and they do not behave as small, individual "waves." Yes, all the photons are in phase, but only because they are part of a single plane-waves.

The light from a laser is basically a single, very powerful light wave. Single waves are always in phase with themselves, but it's misleading to imply that a single plane-wave or sphere-wave is something called an "in phase" wave. Laser light could more accurately be called "pointsource" light. Sphere waves or plane waves behave as if they were emitted from a single tiny point. The physics term for this is "spatially coherent" light. Light from light bulbs, flames, the sun, etc. are the opposite, and are called "extended-source" light. Extended-source light comes from a wide source, not from a point-source, and the waves coming from different parts of the source will cross each other. Starlight and the light from arc welders is "point-source" light and is quite similar to laser light. Light from arc-welders and from distant stars has a higher spatial coherence than light from most everyday light sources. (Note: the sun is a star, correctly implying that light becomes more and more spatially coherent as it moves far from its source. This is a clue as to the REAL reason that lasers give spatially coherent light! (See below)


Light from most lasers is not parallel light. However, if laser light is passed through the correct lenses, it can be formed into a tight, parallel beam. The same is not true for light from an ordinary light bulb. If light from a light bulb were passed through the same lenses, it would form a spreading beam, and an image of the lightbulb would be projected into the distance. Laser light can form beams because a laser is a pointsource, and when you project the image of a pointsource into the distance, you form a narrow parallel beam! However, it is simply wrong to state that laser light is inherently parallel light. Laser light can be FORMED INTO parallel light, while the light from ordinary sources cannot.

Most types of lasers actually emit spreading, non-parallel light. Lasers in CD players and in "laser pointers" are semiconductor diode lasers. They create cone-shaped light beams, and if a parallel beam is desired, they require a focusing lens. The same is true for the lasers in inexpensive "laser pointers." Take apart an old laser-pointer, and you'll find the plastic lens in front of the diode laser inside.

Classroom "HeNe" lasers also create spreading light. The laser tube within a typical classroom laser contains at least one curved mirror (called a "confocal" arrangement,) and it creates light in the form of a spreading cone. It's a little-known fact that manufacturers of classroom lasers traditionally place a convex lens on the end of their laser tubes in order to shape the spreading light into a parallel beam. While it's true that a narrow beam is convenient, I suspect that part of their reason is to force the laser to fit our stereotype that all lasers produce thin, narrow light beams. The manufacturers could save money by selling "real" lensless laser tubes having spreading beams. But customers would complain, wouldn't they? We have been brought up to believe that laser light is parallel light.


In-phase emission causes the AMPLIFICATION of light, it doesn't cause coherent light. Because the atoms emit light in phase with incoming light, they will amplify the light, but they amplify incoherent light too, and they don't make it coherent. The coherence of laser light has another source... Laser light has two main characteristics: it is "monochromatic" or very pure in frequency (this also is called "temporally coherent.") Laser light also has a point-source character of sphere waves and plane waves (also called "spatially coherent.")

Even fairly advanced textbooks fail to give the real reason why laser light is spatially coherent. They usually point out that the laser's atoms all emit their light in phase, and pretend that this leads to spatial coherence. Wrong. It is true that the fluorescing atoms in a laser all emit light that's in-phase with the waves already traveling between the mirrors. But the in-phase emission only creates amplification of the traveling waves, it does not create spatially coherent light. For example, if you were to feed incoherent light into a HeNe laser tube, the atoms would emit in-phase waves, and the laser would amplify the light. But the brighter light would still be incoherent! Lasers certainly can amplify the COHERENT wave which is trapped between their mirrors. But how did the light within the laser get to be coherent in the first place?

Lasers create coherent light because of their mirrors.

The mirrors in a laser form a resonant cavity which preserves coherent light while rejecting incoherent light. How does it work? Imagine a simplified laser having flat, parallel mirrors. As light bounces between the mirrors, the light "thinks" that it's traveling down an infinitely long "virtual tunnel". (Have you ever held up two mirrors facing each other? Then you've seen this infinite tunnel.) When a laser is first turned on, it fluoresces; it emits light which is not coherent. Different random light waves start out from different parts of the laser. After a few thousand mirror bounces, all the waves have added and subtracted to form just one single wave. In the case of flat-mirror lasers, this wave is a nearly perfect plane wave. A single plane wave is coherent (to be incoherent, you must have at least two different waves.)

This can be a bit confusing. After all, the individual atoms each emit a wave. Don't all these waves add up to messy incoherent light? No. The in-phase emission preserves coherence as it amplifies. It's true that each atom emits light waves in all directions. However, these sideways waves cancel each other out, and only the waves that travel in the same direction as the incoming light will be preserved. It's as if the atoms "know" which direction to send out a beam. But in reality, the atoms don't know this. Instead, they just emit a light wave which is in phase with the incoming light, and for this reason the wave from the atom will cancel out everywhere except in a line with the incoming light. If the light in a laser were ALREADY coherent, then the atoms will amplify it but won't make it more coherent. The coherence comes from the great distance that the light has traveled as it bounced between the mirrors.

A similar thing happens with starlight: starlight is coherent! Starlight travels far from its original source and all the waves add up to form a wave with a single wavefront. Light from distant stars is spatially coherent, even though sunlight is not, yet the sun is a star. The farther the light travels from its source, the more it approaches the shape of a perfect plane wave. And a perfect plane wave is perfectly coherent. Laser light is spatially coherent because, among other things, the bouncing light has traveled millions of miles between mirrors, and all the various competing waves have melded together to form a single pure plane-wave or sphere-wave.

P.S. The pure color (monochrome) laser light is ALSO created by the mirrors. Huh? Yes, but the reason for this is not totally straightforward (and it's quite a bit beyond the K-6 level of these webpages!)

The two mirrors of a laser can trap a standing wave of light. The space between the mirrors is like the string of a guitar: there can be a fundamental wave, or overtone waves, or complicated waves which are a mixture of these. But waves of non-overtone frequencies cannot exist between the mirrors. Since the distance between the crests of a light wave is very small, LOTS of different overtones can fit between the mirrors, and each overtone is a slightly-different pure color of light. Light from a neon sign is reddish, but it doesn't have the extreme purity of laser light. Now for the weird part: when a Helium-Neon laser first operates, many different overtones of red light are amplified and the beam contains many slightly-different colors of red at the same time. It's not yet monochromatic. As time goes on, some of these colors are amplified a bit more than others, and this uses up the available energy coming from the power supply. In other words, the different waves start competing for limited resources! Just one wave "wins" in the end, and all of the other overtones drop out of the running. The laser light is not just red light. Instead it is a SINGLE PURE OVERTONE-WAVE, a pure frequency where the string of waves just perfectly fits in the space between the two mirrors. Change the spacing of the laser's mirrors, and you change the frequency of the light.

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